Method for making xerographic plates



March 3 1970 YuEN-sHENG cHlANG ETAL 3,498,835

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March 3, 1970 YuEN-SHENG cHlANG ET AL 3,498,835

METHOD FOR MAKING XERVOGRAPHIC PLATES 4 isheets-sheet s Filed June 28,1966 mvENToR.

YuEN-SHENG CHIANG PETER L. DE PERRO SAMUEL wEl-HSING ING By M ATTORNEY:t f ooo@ 08m 08m oog, ooNm com co2. 8E. ooo 08m 8N@ com oovm ooo N8.QQ. dbmz m woo 3355 mbj eoo lO-O. woo.

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55328965. Nvo. ovo.

March 3, 1970 YuEN-sl-IENG CHIANG ETAI- 3,498,835

' :muon Fon MAKING xEnoGRAPHIc PLATES 4 Sheets-Shegt 4.

Filed June 28, 1966 000m OE ODS. GCIl INVENTOR. YUEN-SHENG CHIANG PETERL. DE PERRO SAMUEL WE|'HSING ING` JR 8" m j ATTORNEY United StatesPatent O 3,498,835 METHOD FR MAKING XERGRAPHIC PLATES Yuen-Sheng Chiang,Peter L. DePerro, and Samuel Wei- Hsing lng, Jr., Webster, N.Y.,assignors to Xerox Corporation, Rochester, N.Y., a corporation of NewYork Filed .lune 28, i966, Ser. No. 561,239 Int. Cl. C23c 11/00, 13/02U.S. Cl. 117-227 9 Claims ABSTRACT F THE DISCLUSURE This inventionrelates in general to Xerography and in particular to xerographicplates, a xerographic process using such plates and to a process for theproduction of such plates. More specically, the invention relates to anew xerographic plate comprising a relatively conductive backing havingon at least one surface thereof a dye-sensitized coating comprising asubstantially homogeneous mixture of vitreous selenium and an organicdye sensitizer.

In the xerographic process as described in U.S. 2,297,- 691 to C. F.Carlson, a base plate of relatively low electrical resistance such asmetal, paper, etc., having a photoconductive insulating surface coatedthereon, is electrostatically charged in the dark.'The charged coatingis then exposed to a light image. The charges leak off rapidly to thebase plate in proportion to the intensity of light to which any givenarea is exposed; the charge being substantially retained lin non-exposedareas. After such exposure, the coatingis contacted with electroscopicmarking particles in the dark. These particles adhere to the areas wherethe electrostatic charges remain forming a powder image corresponding tothe electro-static image. The powder image can then be transferred to asheet of transfer material resulting in a positive or negative print, asthe case may be, having excellent detail and quality. Alternatively,when the base plate is relatively inexpensive as in the case of paper,it may be desirable to x the powder image directly to the plate itself.

As discussed in Carlson, photoconductive insulating coatings compriseanthracene, sulfur, or various mixtures of these -materials such assulfur with selenium, etc., to thereby form uniform amorphous coatingson the base material. These materials have a sensitivity largely limitedto the shorter wavelengths and have a further limitation of being onlyslightly light-sensitive. Consequently, there has been an urgent needfor improved photoconductive insulating materials.

The discovery of the photoconductive insulating properties of highlypurified vitreous selenium has resulted in this material becoming thestandard in commercial xerography. The photographic speed of thismaterial is many times that of the prior art photoconductive insulatingmaterials. Such a plate is characterized by being capable of receiving asatisfactory electrostatic charge and selectively dissipating such acharge when exposed to a light pattern. However, vitreous seleniumsuffers from the serious defect that its spectral response is verylargely limited to the blue or near ultra-violet portion of thespectrum.

Now, in accordance with the present invention, it has been found that animproved xerographic plateV having ICC increased spectral sensitivitycan be prepared by homogeneously incorporating in the photoconductiveinsulating coating a minor amount of an organic dye sensitizer. Theplates as thus modied are characterized by a broader range of spectralsensitivity, particularly toward the red end of the spectrum.

In general, the range of concentration or proportion of organic dyesensitizers in the selenium photoconductive layer includes quantities upto and including about 5% by weight of the vitreous selenium-organic dyesensitizer mixture and, preferably lies in the range of about 0.001 toabout 1% by weight. At percentages substantially above 5%, manysensitizers tend to segregate to the photoconductive insulator surfacethereby substantially altering the desirable physical characteristics ofthat layer.

The new and improved xerographic plates of the present invention arepreferably prepared by a co-evaporation technique. By using thisapproach, as hereinafter disclosed, the organic dye sensitizer isuniformly and homogeneously dispersed throughout the depositedphotoconductive layer. In addition to the increase in spectral response,since a homogeneous layer is deposited its sensitivity is uniform overits entire applicable surface as well as throughout the depth of thephotoconductive layer. This latter result is of i-mportance because asubstantial proportion of the incident light is absorbed and uniformsensitivity can only be obtained if the composition of thephotosensitive layer is uniform. This is a distinct advantage over asurface sensitization where the sensitivity of the surface is vastlydiilerent from the sensitivity of the internal portions. Furthermore,erosion of the uppermost portion will not destroy the effectiveness ofthe homogeneous layer as it would when a surface sensitized layer isused. These advantages are a direct result of the utilization of theherein disclosed co-evaporation techniques.

Selenium and the dye sensitizer are placed in separate sources in a highvacuum vessel and are heated to volatize their contents at a desireddeposition ratio. When a steady state equilibrium is reached, a shutterseparating the sources from the conductive backing is removed and theevaporation is permitted to continue until the desired depth ofphotoconductive insulator layer is deposited upon the conductivesubstrate. The shutter is then reinserted between the containers and theconductive backing to prevent further deposition, the heating units areturned olf and the entire apparatus cooled to roo-m temperature.

FIGURE l is an illustrative example of the type of apparatus which canbe used in preparing the improved xerographic plates of the presentinvention. A clean conductive plate 1 is attached to a temperaturecontrolled platen 2 a xed distance above a selenium container 3. Thetemperature of the plate may be controlled in any suitable manner, suchas by the passage of water through a suitable conduit 8 in the platen,etc. The plate temperature is maintained at a level whereby vitreousselenium is deposited during the deposition process. Thus, temperatureson the order of C. may be used, provided the time of deposition isrelatively short; whereas lower temperatures are most commonly used withlonger periods of deposition. Preferably, the temperature of the plateis held between about 20-75 C. Above 75 C. for relatively long periodsof deposition, there is obtained an increasing proportion of crystallinerather than vitreous selenium; and below about 20 C., it has been foundthat the xerographic plates so produced do not have the normal anddesirable photoconductive insulating properties.

A temperature controlled container 3 holding purified selenium is placedin the bottom of the deposition vessel. A plurality of .temperaturecontrolled containers 4 each holding a quantity of an organic dyesensitizer are placed intermediate the selenium container 3 and theconductive backing 1. The mouths of the organic dye holding containers 4are approximately 5-15 inches from the conductive backing 1. Thecontainers 4 are supported by any suitable movable means 5.

A vacuum is drawn on the vessel by means of a high vacuum pump (notshown) connected to the deposition vessel by conduit 6 and with movableshutter 7 in the closed position, the respective containers are heatedto the appropriate temperatures until a steady state equilibriumcondition is reached. The pressure in the vessel may range from 1to'10-3 microns of mercury. The selenium is maintained at a temperatureabove its melting point and at a point where its vapor pressure issufficient to provide substantial deposition on the conductive backing.At the other extreme, the selenium should not be maintained at atemperature which provides (1) too great a rate of evaporation so thatthe dye material is not homogeneously dispersed through the depositedlayer or (2) insufficient dye is trapped in the deposited layer.Temperatures of between 230-300 C., preferably about 270 C., aresuicient to give the desired deposition rate and, in conjunction withthe appropriate temperature conditions for the organic dye containers,to attain the desired sensitizedselenium layer. The organic dye holdersare held at a temperature whereby the dye is converted into the vaporousstate in amounts suicient to provide homogeneous dispersion of thatmaterial throughout the deposited selenium layer. The temperature shouldnot be maintained at such a high level that there is a possibility thatthe dye might decompose. Temperatures on the order of about 190-220 C.,preferably about 200 C., are suicient for most organic dyes. Theselection of a particular temperature at which to hold the organic dyematerial depends on many factors such as the decomposition point, thevapor pressure, the ambient pressure, etc., and it is conceivable thatfor a particular organic dye the appropriate temperature may falloutside the range heretofore specified. It should be understood,however, that the selected temperature is determined on the basis of theaforesaid physical characteristics rather than merely operating Withinthe temperature range previously indicated as being suicient for mostdyes.-

Suicient selenium and organic dye should be used so an equilibriumcondition may be maintained throughout deposition. In this manner, auniformly homogeneous sensitized layer is deposited on the conductivebacking. In other words, excess selenium and organic dye should beplaced in the container so that at no time during the deposition processwill the vaporous components be present without a corresponding solid orliquid phase. Under equilibrium conditions, the selenium may be ineither the liquid or solid phase, depending on the ambient conditions,Whereas the organic dye will normally be in the solid phase. Ifinsufficient quantities are utilized the equilibrium condition would bedestroyed and the deposited layer would not be uniformly homogeneous.

The rate of deposition is limited only by the rates of evaporation ofthe respective materials at ambient conditions. In essence, however, thedeposition rate is limited by the rate of evaporation of the moresensitive material, namely, the organic dye. In general, the dyes have arelatively low vapor pressure even at elevated temperatures. Increasesto yet higher temperatures is limited by fear of decomposition. Such adecomposition would, of course, obviate the desirable results asheretofore set forth. Accordingly, the rate of evaporation of theselenium is limited so that suicient dye material is homogeneouslytrapped throughout the photoconductive insulator layer as it isdeposited upon the conductive backing. While rates of -20 microns perhour are easily obtainable, it is contemplated that under appropriateconditions with suitable apparatus any desired rate of deposition can beobtained. The advantageous results set forth in this application are notentirely dependent upon the optimum rate of deposition; so long as thespectral response of the xerographic plate is increased, it should beunderstood that 4 such plates, and the coevaporation processes used toobtain such plates, are within the scope of this invention regardless ofthe rate of deposition of the photoconductive insulating material.

The concentration of the sensitizing material in the resultant seleniumphotoconductive insulator layer depends, inter alia, on the followingvariables: (1) the temperature of the conductive backing, (2) thetemperature of the selenium holder, (3) the temperature of thesensitizing material holders, (4) the relative openings of therespective holders, and (5) the distance the holders are placed from thebase plate. Thus, for a fixed distance between the selenium holder andthe conductive backing and all ambient conditions being constant, theproportion of sensitizer in the deposited layer can be increased by (1)raising the temperature of the sensitizer holders so as to increase therate of evaporation of the organic material, (2) moving the dye holderscloser to the conductive backing, or (3) increasing the number of dyeholders, etc.

The general scope and nature of the invention having been set forth, thefollowing example is given as a typical illustration of a method bywhich the desired xerographic plates may be prepared. A cleaned aluminumplate approximately 3 inches wide, 3 inches long, and 1/8 inch thick isattached to a temperature controlled platen about 20 inches above aselenium container. This distance remains ixed during the depositionprocess. The temperature of this aluminum plate is held at about 50 C.

A temperature controlled aluminum container having a circular mouth 21/2inches in diameter holding approximately 20 grams of puried selenium isplaced in the bottom of the deposition vessel. Two temperaturecontrolled aluminum containers each containing about 2 grams of RoseBengal free-acid are placed intermediate the selenium container and thealuminum plate with the mouths of the containers being approximately l0inches from the aluminum plate.

Air is evacuated from the air tight vessel until a pressure of about10-2 micron is reached. With the shutter in the closed position, theselenium container is heated to 270 C. and the Rose Bengal containersare heated to 200 C. After suflicient time has passed for the materialsto reach an equilibrium state, the shutter is removed and the depositionis permitted to take place. Deposition continued for approximately 2hours. The homogeneous Rose Bengal sensitized-selenium layer so obtainedis approximately 20 microns thick.

Any suitable dye sensitizers may be used in the practice of thisinvention. Typical sensitizers include Rhodamine B, Crystal Violet andthose having the formula:

wherein X is either a halogen or a hydrogen atom, m is an integer equalto 0, 1, 2, or 3; and n is an integer equal to 0, l, 2, 3, or 4.

Rhodamine B (C.I. 45170) has the following structure:

Crystal Violet (C I. 42555) has the following structure: (Fieser &Fieser, Organic Chemistry, Third Edition, page 893, 1956, Library ofCongress Card No. 56-6'691):

I CGN-cumm- N(CHa)z The following organic dye materials are consideredexemplary of compounds falling within the scope of the aforesaid generalformula:

o Ho -o l C/ ooorr Rose Bengal-free acid (C I. 45440) COOH Br Br Ho W-oBr \C/ Br I C1 COOH Ol Cl Phloxine B-free acid (CJ. 45410) Br o Br Ho -oBr Br COOH Tetrabromo-uorescein (C.I. 45380A) (Eosin Yellow) o1 (In o Ho-o \Cf @Goor-r Dichloro-uorescein The dye sensitizers may be used singlyor in combination to enhance the spectral response of thephotoconductive layer to electromagnetic radiation in the visibleportion of the spectrum. For example, a dye which enhances response inthe 400G-6000 A. wavelength portion of the spectrum may be incorporatedin conjunction with a dye that enhances response in the 6000-8000 A.Wavelength portion.

The sensitivity of the xerographic plates prepared in accordance withthe coevaporation process is measured by a photo-discharge technique.The photoconductive insulator coated substrate is placed on a movablesupport, uniformly electrostatically charged by an air corona, and thenexposed to monochromatic light. In all measurements the light isangularly incident upon the photoconductor surface opposite to that ofthe supporting substrate -which is held at zero potential. The voltagedecay of the photoconductor insulator material upon phot-on excitationis monitored yby an electrostatic probe. Because of the insulatingnature of the photoconductor insulator film and the non-injectingcharacteristics of the probe, the dark decay rate is slow; in any event,the voltage decay is measured within seconds after the electro staticcharge is placed uniformly over the photoconductor insulator surface.The signal received -by the probe is fed into an electrometer and theoutput of the electrometer is fed into a diierentiator. Thedifferentiator signal is recorded; its amplitude being proportional tothe voltage decay rate. The quantum efficiency, a measure of thesensitivity of the xerographic plate, is derived from the number ofelectrons generated per ,absorbed photon. The number of electronsgenerated per second can be calculated from the initial voltage decayrate (dv/dto). The photon flux is calibrated against a standardphoto-cell with the actual photon absorption being the incident photoniiux minus the reflected photon flux (usually about 25% at allwavelengths). The calculations made are based on the assumption that theselenium plate, used as the standard, has a `quantum efciency of oneunder illumination of light of 4,000 A. wavelength (Paul I.Regensberger, J. of Applied Physics, 35, p. 1863, 1964).

'FIGURE 2 is a graph showing the spectral response curves of a seleniumxerographic plate contrasted with the Rose Bengal sensitized seleniumplate produced in accordance with the above example. The RoseBengalselenium curve actually represents a family of curves forsensitized layers containing about 0.001 to about 0.1% sensitizer.

FIGURES 3 8 represent similar graphs showing the contrast in spectralresponse between selenium Xerographic plates and sensitized platescontaining, respectively, Phloxine B, tetrabromo-uorescein,dichloroiluorescein, uorescein, Rhodamine B, and Crystal Violet. Theincorporation of these sensitizers into the photoconductive insulatingmaterial is conducted in accordance with the procedures previously setforth.

FIGURES 6, 7, and 8 show that while the sensitivity of selenium is notappreciably increased up to wavelengths of approximately 6,000 A., theaddition of fluorescein, Rhodamine B, or Crystal Violet materiallyincreases the sensitivity at wavelengths greater than 6,000 A. Thesensitivity of the unsensitized selenium in this latter portion of thevisible spectrum is so low that it is off the graph. In other words, theaddition of the electron donor organic dyes increases the sensiti-vityof the selenium plate in the red portion of the spectrum by severalorders of magnitude.

The selenium used in the preparation of xerographic plates should befree of impurities which adversely effect its ability to holdelectrostatic charges, that is, by forming conducting paths in the iilmor promoting the formation of conducting hexagonal selenium so thatelectrostatic charges leak oil rapidly even in the dark andelectrostatic deposition of powder or other finely-divided material cannot be obtained. Preferably, there should be used vitreous seleniumavailable in pellet form IAT inch to Ms inch size under the name ARQ(ammonia reduced in quartz from selenium oxide) as manufactured, forthis grade of selenium is essentially pure, containing less than aboutparts per million of impurities. If puried, other grades of selenium,i.e., DDQ (double distilled in quartz) and CCR, commercial grade) asmanufactured, can likewise be employed in the process discl-o-sedherein. Procedures to purify these grades of selenium are well known inthe art; accordingly, they will not be discussed here.

1Other materials such as alloy of selenium with minor amounts ofarsenic, tellurium, sulfur, etc., can also be used as thephotoconductive insulating material in the practice of the presentinvention. If such alloys are used, each ingredient, including theorganic dye sensitizer, can be evaporated from separate vessels, or, inthe alternative, the selenium alloy can be formed prior to thesensitization deposition and thus will be evaporated from a singlesource. While the nature of the selenium layer has been described asvitreous, the exact molecular structure it not known, the term beingused as descriptive of its physical appearance. It is believed that theselenium is present substantially in an amorphous form containing minorproportions of a crystalline form of selenium, although it is notdesired to restrict this invention to the presence of such a mixture offorms. lt is, therefore, to be understood that the various crystallineor amorphous structures included in the vitreous-appearing form ofselenium are likewise to be included in the term vitreous as used hereinand in the claims. It is likewise to be understood that the termselenium includes not only pure selenium but also selenium that may bemodied by a controlled amount of an additive, such as noted above, thatis consistent lwith the retention of useful photo-conducting properties.

A conductive backing is usually required for xerographic plates andmetal forms the most suitable material. However, a high conductivity isnot required and almost any structurally satisfactory material which ismore conductive than the organic dye sensitized selenium layer can beused. Materials having electrical resistivities about 101 ohm-cm. aregenerally satisfactory for the base plate of this invention although itis more desirable to use materials of less than about 105 ohm-cm, Anygross surface irregularities, i.e., burns, tool marks, are removed frornthe base plate by grinding or polishing although it unnecessary topolish the plate until it has a mirror-like surface. The conductivebacking surface is cleaned before coating with the dyesensitized-selenium in order to remove grease, dirt, and otherimpurities which might prevent lm adherence of the coating to the baseplate. This is readily accomplished by washing the plate with anysuitable alkali cleaner or with a hydrocarbon solvent such as benzene,followed by rinsing and drying. Suitable backing materials are aluminum,glass having a conductive coating thereon as of tin oxide, (NESA glass)or aluminum, stainless steel, nickel, chromium, zinc, and steel.

Also, conductive plastic, conductively coated paper, or other web ornlm-like member may be used as the conductive supporting surface asdesired. It is to be understood that the backing members elected forthis plate may be in the form of a flat plate or may equally be in theform of a cylinder flexible sheet, or other member having a surfacesuitable for the xerographic process.

The xerographic member of the present invention may be used as thelight-sensitive member in any of the regular xerographic processes. Themember is electrically charged to a potential of the order of about 100to 800 volts by any method well known in the art. The charged member isthen exposed to a light image whereby there is a selective dissipationof the electrostatic charge. The resulting latent electrostatic imagecan be developed, i.e., mad@ YSbie, by treatment with an electroscopicmaterial and, optionally, the developed image can be transferred to asupport member to yield a xerographic print. Modications and variationsof this process need not be considered as they are well known to thoseskilled in this art.

Themanner of electrically charging the xerographic member is not at allcritical. The charge can be either positive or negative in polarity,Excellent xerographic copies have been obtained when the xerographicplate is sensitized with an electron acceptor organic dye, such as RoseBengal, Phloxine B, tetrabromo-uorescein, and dichloro-fluorescein, andthe plate is positively charged. Additionally, excellent xerographiccopies have been obtained when the plate is sensitized with electrondonor organic dyes, such as fluorescein, Rhodamine B, and CrystalViolet, and the plate is negatively charged. A tungsten light is thesource of the imaging light in the production of each xerographic copy.

It is recognized that the organic dye sensitizers which are shown bythis application to improve the spectral response of xerographic platesare Iwell known in the art of sensitizing zinc oxide photoconductiveinsulator layers. However, prior to this application such materials havenot been known to be used to sensitize vitreous seleniumphoto-conductive layers. Additionally, the sensitization of zinc oxidebinder plates is considered to be a sensitization only on the surface ofthe zinc oxide particles resulting in a non-uniform dispersion of thesensitizer throughout the photoconductive layer. This is in contrast tothe present invention wherein the sensitizing material is uniformly andhomogeneously dispersed throughout the selenium layer by means of thecoevaporation technique.

While the invention has been described with reference to preferredembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made without departingfrom the true spirit and scope of the invention.

Further, provided the advantageous results of this invention are notadversely affected, additional operations may lbe performed to achievethe herein disclosed results, or in certain circumstances, certainoperations may be deleted as will be apparent to those skilled in theart. The apparatus herein disclosed may beV modified in numerous ways,to once again, achieve the increase in spectral response of the vitreousselenium xerographic plates. All such additions, deletions,modifications, etc., are considered to be within the scope of thepresent invention.

What is claimed is:

1. The process of producing a xerographic plate comprisingco-evaporating selenium and at least one organic dye sensitizer anddepositing the vaporized selenium and organic dye onto an electricallyconductive backing member.

2. The process of claim 1 wherein said materials are deposited after asteady state equilibrium condition is reached.

3. The process of producing a xerographic plate comprisingco-evaporating selenium and at least one organic dye sensitizer,establishing a steady state equilibrium condition between the solid andvaporous phases of said selenium and said organic dye, and depositing ahornogeneous dispersion of vitreous selenium and said organic dyesensitizer upon a conductive backing member.

4. The process of claim 3 further including the step of terminatingdeposition while said equilibrium condition exists.

5. The process of claim 3 wherein the selenium and said organic dyesensitizer are evaporated under a pressure equivalent to between about land about 103 microns of mercury.

6. The process of claim 3 wherein the selenium is evaporated from asource held at a temperature between about 230-300o C.

7. The process of claim 3` wherein said organic dye 9 10 sensitizer isevaporated from a source held at a tempera- References Cited ture belowthe decomposition point of said organic dye, UNITED STATES PATENTS saidtemperature being suicient to cause conversion of said organic dye insensitizing quantities from its initial 312221218 12/1965 Beltzer117-227 X state to the vaporous state. 5 FOREIGN PATENTS 8. The processof claim 7 wherein said organic dye 830 348 3/1960 G B source is held ata temperature of between about 190 reat mam' and about 220 C. WILLIAM L.IARVIS, Primary Examiner 9. The process of claim 3 wherein theconductive backing member is held at a temperature between about 10 U-S'C1' X'R'

